We have developed novel methods that allow us to reproducibly obtain an exuberant outgrowth of endothelial cells from blood, starting with reasonable quantities of peripheral blood. This outgrowth has the characteristics of healthy, quiescent microvascular endothelial cells. We propose that these will be of use in various biomedical, biomedical engineering, and tissue engineering applications. TO test this, we will conduct three proof-on-concept studies to document the utility of the endothelial outgrowth from blood. [1] To illustrate the feasibility of using these cells for diagnosis for endothelial-based disorders, we will assay them for thrombomodulin activity in normals, in persons known to have non-functional mutant thrombomodulin, and in patients with thrombotic diathesis. [2] To illustrate the feasibility of using them to coat devices, we will document that sufficient numbers can be obtained to line the lumen of vascular grafts with a confluent of layer of endothelial cells. [3] To illustrate their utility for gene therapy purposes, we will document that we can express cDNA for a modified coagulation factor VIII (marked and B domainless) and thereafter further expand the transfected/transduced endothelial cell population.